Most biometrics used today which rely on individual characteristics, including voice recognition, fingerprint, retinal and facial recognition, and are based on recognizable visible or audible characteristics, may be vulnerable to falsification, because technology allowing accurate copying and modification of such characteristics, i.e., CCD cameras, 3D printers and digital recordings, is widely used and available. Instruments for copying or modifying modified electrical signals, however, are not widely available. Generating an electrocardiograph (or ECG) requires a living and physically present individual at the identification site, making it more difficult to falsify. In addition, an ECG signal has all the properties of a secure biometric: universality, measurability, uniqueness, and permanence.
ECG signals can be transmitted for recording using conventional surface electrodes, usually mounted on the subject's chest. ECG signals are made up of several components representative of different functional stages during each heart beat, and projected according to the electric orientation of the generating tissues. There is a wide variety of subject-specific detail in electro-cardiologic signals due to variations in the heart tissue structure and orientation among individuals.
The ECG has been studied extensively as a potential biometric, but the inconvenience of the 12-lead detection, which was required, and insufficient reliability have inhibited widespread adoption. While it may be possible to use fewer leads, and to find unique portions of an ECG signal or a derived ECG signal, it has been difficult to find a reliable identifier using ECG signals where the subject's pulse rate varies (which it does on a continuous basis).
This electrical signal includes a sequence of PQRST complexes, and most PQRST sequences are not uniform. The time interval between two consecutive R signal peaks, referred to as an R-R interval, corresponds to a heart pulse, with a rate that normally lies in a range of 60-90 beats per minute (bpm). The P signal corresponds to atrial depolarization (right side, depolarizing first, followed by left side). The larger QRS complex corresponds to depolarization of the ventricles and repolarization of the atria. The T signal corresponds to repolarization of the ventricles. A weaker U signal occasionally appears.
A “wave” comprises a curve covering at least one complete component (P, Q, R, S and/or T). A time increment with a straight line amplitude extending between two consecutive signals, for example, from the end of an S wave to the beginning of an immediately following T wave, is referred to as a “segment.” A time increment that includes at least one wave, with a graph that is at least partly curved, for example, from the beginning of a Q wave to the end of an S wave, is referred to as an “interval.”
The QRS time interval, normally of temporal length 50-100 milli seconds (msec), represents conduction time from initiation of ventricular depolarization until the end of ventricular depolarization, and includes spread of the electrical impulse through the ventricular muscle. The P wave signal is normally gently rounded, and has a temporal length≈50-110 msec. A QRS interval greater than about 120 msec often indicates ventricular arrhythmia or a block of one of the bundles.
When the ECG was first developed for clinical use, a low-pass filter was implemented to eliminate the high-frequency “noise” in the signal to make it easier to assess the shape of the various waves and the length of the intervals in the displayed output. The high-frequency component of the QRS complex (HF-QRS) in the ECG signal—which is a distinct part of the ECG signal and is not the same as the ECG signal itself—was largely forgotten until researchers at NASA demonstrated it could be used to more precisely determine the health of the heart tissue, especially in relation to the electrical conducting system, as it more precisely represents the unique electrical conducting system of each person's heart. See U.S. Pat. Nos. 8,924,736; 7,539,535, both incorporated by reference. The volume of data present in the HF-QRS signal is significantly more than is present in standard low-frequency ECGs used today, and several patterns that NASA identified in the data have been shown to be relatively stable from month to month (T. T. Schlegel, et al. “Real-time 12-lead high-frequency QRS electrocardiography for enhanced detection of myocardial ischemia and coronary artery disease” Mayo Clin Proc, March 2004, Vol 79, pp. 339-50).
The number of features in the HF-QRS signal, and the amount of data that can be extracted from it, is considerably greater than other biometric identification measures currently monitor, for example, as compared with fingerprints, faces, conventional ECG readings, or retinal blood vessel patterns. Using the HF-QRS signal as a biometric therefore can permit making more reliable identification.